The quality of HIV intervention research is dependent upon the reliable and valid measurement of behavior change. To the extent that study outcomes are demonstrated with measures that meet high scientific standards for reliability, validity, and conceptual relevance, we can have confidence in the behavior change effects produced by interventions. This core will support and stimulate basic research on assessment methodology through four functions: 1. Develop, pilot test, and evaluate existing data sets, and provide consultation of CAIR investigators in the areas of multidomain and multimethod assessment of sexual and substance use practices that confer risk for HIV infection, focusing particularly on measures to corroborate self-reports of behavior change and reliability and validity of these measures; 2. Develop, pilot test, and provide consultation to CAIR investigators on methods to tailor measure content and administration format to ensure cultural appropriateness for ethnic minority and low-reading level individuals; 3. Construct assessment measures related to intervention conceptual components in order to enrich opportunities to more precisely identify characteristics of interventions and mediators responsible for their behavior change effects; and 4. Establish a centralized system for efficient cohort tracking and followup location that can be used across multiple CAIR intervention outcome studies. This core will bring together a team of scientists experienced in behavioral, quantitative, and qualitative assessment research methods. The core will support the development, refinement, and adaptation of assessment methodologies and measures for use in existing and new CAIR investigators, will provide support and consultation to new investigators, and will conduct activities to stimulate improvements in research assessment methodologies.